An international length comparison using vacuum comparators and a photoelectric incremental encoder as transfer standard

The German national metrology institute, a Japanese company, and a German company have conducted an international length comparison. A photoelectric incremental encoder with a measurement length of 280 mm served as transfer standard. The traceability of measurement results to the SI unit of the metre was uniformly realized through iodine-stabilized lasers and, for the first time all length comparators involved used vacuum interferometry. This made it possible to reach an agreement of calibration results of 5.5 nm over the total length of 280 mm, which corresponds to a relative agreement of 2 × 10−8. The short-range deviations between the calibration results can be characterized with a standard deviation of σ = 0.4 nm. The results attained are consistent with the calculated measurement uncertainty values and are a distinct improvement over previously conducted length comparisons.